weight of the put. The munitions experts back in pre-revolutionary times came up with a series of tests to see who would be the best for moving munitions quickly on the battle field. This helped with arranging the cannons equidistance apart from munitions piles, etc.
The story of the Javelin is really interesting if you want to get into it.
And if you stayed 'til the end you now know that I made all of that up. Have a great night!